The invention finds applications, in particular, in the automotive sector, for example in systems for electronic control of an actuator, such as a device for electronic control of the throttle (or ETC device, the abbreviation standing for “Electronic Throttle Control”) or of the burnt gases recirculation valve (or EGR device, the abbreviation standing for “Exhaust Gas Recirculation”), or of any other valve used in engine monitoring or the like, or more generally of any other item of equipment actuated by electric motor, such as a window winder, for example.
The control of inductive loads by pulse width modulation may in general be performed by a switching structure such as an H-bridge. This structure comprises four power switches, namely two “high” switches on the side of a positive power supply and two “low” switches on the side of a negative power supply or the ground.
A first pair formed of a first high switch and of a first low switch enables, when they are both closed, current to flow in the load in a certain direction. A second pair formed of the other high switch and of the other low switch enables conversely, when they are both closed, current to flow in the load in the opposite direction. The two low switches, or the two high switches, when they are both closed, allow the flow of a freewheeling current.
Each switch generally comprises a power MOS transistor (the acronym standing for “Metal Oxide Semiconductor”. A sequence of analog control signals for the four MOS transistors is produced, according to a determined strategy, on the basis of a setpoint control signal. This setpoint control signal can be pulse width modulated, with a duty ratio making it possible to control the quantity of current injected into the load and therefore, on average, the intensity of the current in the inductive load.
For this purpose, the switching structure is alternately positioned in a certain state in which the flow in the load of a current of determined value is controlled in one or the other direction, and in another state in which a freewheeling current is permitted to flow in the load, through two transistors which are closed.
Gentle variations in voltage and current (called “Slew-Rate”) are effected at the level of the control gates of the MOS transistors, so as to avoid sudden switchings that generate electromagnetic disturbances.
Energy losses at the level of the switches are of two different kinds: static losses, produced through the Joule effect when the switches are closed, and dynamic losses related to the switching of the switches. The former losses are related to the internal resistance RdsON of the MOS transistors. The latter losses are related to the switching speed of the MOS transistors. The slower the variations in current and voltage, the more significant the dynamic losses.
The dynamic losses are essentially localized at the level of the transistors which do not participate in the freewheeling. They depend on the sweep rate (Slew-Rate) in voltage and in current.
In case of a spike in the current absorbed in the load, of insufficient cooling, and/or of too high an ambient temperature, the temperature at the level of the junctions of the MOS transistors, termed the “junction temperature”, may rise beyond acceptable limits, which depend on the technology used.
This is why, in order to limit the rise in the junction temperature of the transistors and to thus avoid their destruction, it is possible to implement a temperature-sensitive current reduction mechanism or TDCR mechanism.
The effect of such a mechanism is to automatically reduce the current in the load when the junction temperature of the “low” transistors exceeds a first threshold (called the warning or “alert” threshold), lower than a second threshold (called the cutout or “Shutdown” threshold) beyond which the flow of current in the MOS transistors is interrupted so as to avoid their destruction. Such a TDCR mechanism intervenes on a maximum permitted value for the current in the load, which is used as high stop in the strategy for controlling the current in the transistors. The permitted maximum current is reduced, and then optionally increased again, in both cases in a linear manner, until a temperature-current equilibrium is reached. At this equilibrium point, the junction temperature and the maximum current permitted in the load are stabilized.